Fluid coke aggregate and electrode



My 3U w67 H, LQEV'NSTEIN 3,322,6@3

FLUID COKE AGGREGATE AND ELECTRODE Filed Aug. 1, 1962 3,322,663 FLUID COKE AGGREGATE AND ELECTRODE Hirsch Loevenstein, Los Angees, Caiif., assigner to Harvey Aluminum, Torrance, Calif., a corporation of California Filed Aug. 1, 1962, Ser. No. 214,137 8 Claims. (Cl. 2041-294) This invention relates to a fluid coke electrode and the process of making this electrode. More particularly, the present invention relates to electrodes comprising fluid coke which can be utilized for obtaining aluminum from its ores and the process of making such electrodes.

In recent years a new method of treatment of crude, heavy hydrocarbon oils is finding more and more application in the liquid fuels industry. By this method the .conversion of the heavy hydrocarbons to lighter fractions is conducted in a fluid bed reactor and, in consequence, the residual coke is not obtained as a solid porous body (delayed coke), but in the form of compact small spheres having in their greatest part particle sizes between 400 and 75 microns (-35 to +200 mesh Tyler).

The product is called, in difference from the old known petroleum coke (delayed coke)-iluid coke.

Many attempts were made to introduce this new type of coke as raw material in the production of electrodes, especially self baking electrodes used on a large scale in the electrolytic reduction of alumina to metal. But, up to date, only an insignificant amount of it is used, in horizontal stud anodes as an admixture to delayed coke in amounts not exceeding 25-35%, and no other successful commercial use has been made of this material as an electrode.

One of the reasons for this lack of c-ommercial success is because the fluid coke consists of very small spheric particles rarely exceeding 850 microns; Whereas a satisfactory electrode paste must contain an appreciable amount (at least 40%) of particles exceeding 990 microns (16 mesh-Tyler).

Another problem which was anticipated in the use of iiuid coke was the expectancy that such electrodes would have a pronounced tendency to dust This problem was expected to be especially severe in self baking electrodes which are produced by `baking an agglomerato which comprises coke particles and pitch, the latter serving as a binder. It is important that the coke resulting from the pitch have about the same reactivity as the coke used for the electrode. If the reactivity of the coked pitch is higher than that of the electrode coke, the electrode will fall into powder or dust because the binder coke will react preferentially with the oxygen formed at the electrode, thus liberating the still unreacted electrode coke particles. Laboratory reactivity tests, by which the dusting tendency of an electrode is normally evaluated, appeared to confirm this expectancy. Baked iiuid coke plugs of anode paste and plugs baked from delayed coke paste (the petroleum coke which has conventionally been used for the production of electrodes) were subjected to the standard laboratory reactivity test which consists of exposing the plugs to a stream of carbon dioxide for 4 hours at 950 C. (which is the normal temperature of the bath in an aluminum cell). The results of this test are indicated in Table 1.

The appearance of the plugs after the test was also different. The delayed coke plug showed a uniform surface while the fluid coke plug showed a large number of unreacted coke particles protruding from the mass.

Still another problem connected with the use of fluid coke for electrodes is associated with the physical structure of the iiuid coke particles. Unlike delayed coke, which has a porous structure which permits the binder to penetrate deep into the particles, fluid coke is non-porous and, even when ground and compressed into briquettes or pellets, retains a compact substantially non-porous structure. Furthermore, it would be expected that particles wetted only on the surface by binder would be unlikely to form, during baking, a uniformly strong electrode which would not break up during use and which would have a conductivity comparable to delayed coke electrodes.

One of the principal objects of the present invention is to provide a fluid coke electrode which may satisfactorily be used in electrolytic cell operation, and the process of producing such an electrode.

Another object of the present invention is to provide a fluid coke paste which may be used in a self baking electrode, and the process of producing this paste.

Still another object of the present invention is to provide a fluid coke electrode comprising an agglomerate having the most suitable particle size distribution for use in electrolytic cells and the process of making this electrode.

A further object of the present invention is to provide an electrode paste which need not be completely desulfurized, and the process of making this electrode.

Other objects and advantages of this invention, it is believed, will be readily apparent from the following detailed description of preferred embodiments thereof when read in connection with the accompanying drawing.

In the drawing:

The single figure is a graph illustrating the preferred granulometry, indicated by line A, of the present invention. The particle size designations used in this graph are the same as those set forth in Table 2 herein.

Briefly, this invention comprehends fluid coke electrodes made from previously calcined iiuid coke pellets or briquettes. The present invention also comprehends a briquetted fluid coke paste having specic particle size limitations and the process of making the paste into an electrode. This process comprises briquetting fluid coke particles, calcining the briquettes, grinding the briquettes such that a composition comprising 33 to 46% total fines, 14 to 26% medium and 37-47% total coarse is obtained. This composition is then admixed with a suitable binder, eg., hard pitch, to produce a paste. This paste may then be compressed and baked to form a conventional electrode or the paste may be used to form a self baking electrode. In a preferred embodiment of the present invention, the proportion of very fines to total lines comprises from about 60% to about 70% by weight. It is also within the scope of the present invention to use previously desulfurized fluid coke. Furthermore, one of the features of the present invention is that the desulfurization need not be carried out to the same extent as that considered necessary by the prior art because it was found that the electrodes have the property of auto-removal of sulfur during baking.

The following specific examples are illustrative of the process and product of the present invention, but it is to be understood that the invention is not to be limited to the details thereof.

Example 1 Fluid coke briquettes produced according tothe process described in detail in my pending U.S. application Ser.

No. 76,585, filed Dec. 19, 1960, and now abandoned, were ground to size according to the specifications set forth in Table 2. This ground product was then mixed with 33.1% hard pitch to form a paste. This paste was then used in a 100,000 amp vertical stud cell. It was found that the anode filled with this paste was ready to be put into operation in twelve days, as compared with the normal prebaking time of about three weeks. This, of course, means a saving of at least a week and a consequent similar gain in metal production of the cell.

The percentage of very fines to the total fines in the aggregate of this example was 69.2%.

Example 2 The 100,000 amp vertical stud cell was continually fed with the paste of Example 1 and alumina was electrolytically reduced therein. It was found that the paste consumption per pound of metal produced was about 0.562 pound. The electrode showed no air burns (which often appear in delayed coke electrodes) and there was substantially no dust formation in the bath. This lack of dust formation was totally unexpected, on the basis of the laboratory tests previously described.

Cores were drilled in the electrode. The properties of these cores are indicated in Table 3.

to be only 0.51%. As previously noted, the pellets used to produce the fluid coke paste contained at least 2% sulfur. On this basis, the paste itself contained at least about 1.34% sulfur. Furthermore, during the baking of the paste, the volatiles of the pitch are driven off. Therefore, the percentage of sulfur in the baked anode should be theoretically even higher than 1.34%. Instead, as mentioned above, the sulfur content was found to be considerably lower than 1.34%, namely, 0.51%. This compares favorably with the sulfur content (0.41%) of the baked electrode prepared from the above-noted low-sulfur delayed coke.

It is believed, although the present invention is not to be considered to be limited by this belief, that the explanation for this reduction in sulfur during baking is that the volatiles of the binder react with the sulfur of the coke. During baking, the paste passes through temperatures ranging from 350 C. to 800 C. It is believed that the reaction first takes place slowly in the temperature range of 350 to 700 C. and then proceeds rapidly in the range of 700 to 800 C. The occurrence of such a reaction was confirmed by placing a wetted lead acetate paper over the surface of the anodes. This paper gradually turned brown because of the release of hydrogen sulfide from the anode. The presence of hydrogen sulfide was also ascertained in the gases escaping under the skirt of the anode.

The discovery of this process of auto-removal of sulfur during baking of electrodes is important because this phenomenon permits the use of pellets with a higher sulfur content than would otherwise be permissible. This of course permits the production of pellets at a lower cost because the desulfurization time of the pellets themselves may be reduced. The fiuid coke composition of the present invention may, of course, be used with equally good results either as a conventional baked electrode or as a self baking electrode.

In practicing the present invention, it has been found that particularly good results are obtained by using a TABLE 3 Real Density Porosity Distance from Apparent Crushing Resistivity Volatiles Surface Density Strength, (ohms/in!) (Percent) p.s.i. Water Hg Ord. Micro Example 3 composition having a particle size distribution within the It has also been found that the initial sulfur content of the fiuid coke paste is less critical than was originally anticipated. To confirm this fact, pellets were produced according to the process described in Example 1, with the exception that desulfurization was stopped when the pellets contained between about 2% and 3% sulfur. These pellets were then formed into a paste as described in Example 1.

This type of paste was used in a self baking electrode. After ten months of continuous operation, samples were taken from the steel studs conducting electricity from the electrode and analyzed for their sulfur content. These samples were taken at the tip` of the stud. The sulfur content of the surface skin, after cleaning with a brush was found to be 0.48%. A sample taken at a distance of 5 to 10 millimeters from the surface was found to have a sulfur content of 0.039%; at a distance of 10 to 15 millimeters, the sulfur content was 0.040%. The corresponding values for a stud in operation for 12 months in a delayed coke electrode, which coke contained less than 1% sulfur, were 0.17%, 0.040%, 0.038% and 0.039%. The surface scale on the stud in the fiuid coke electrode was -found to contain 13.94% sulfur while that on the stud in the delayed coke electrode contained in this case the relatively high amount of 23.33%. The sulfur content of the baked fluid coke electrode was found range described by the line designated A in the graph of the drawing. Furthermore, it has been found desirable to maintain the proportion of very fines, i.e., less than 200 mesh, to the total fines, i.e., less than -65 mesh, within the range of from about 60% to about 70% by weight.

It has also been found that compositions having the following particle size distribution are satisfactory for use in the practice of the present invention: 33-46% total fines, 14-26% medium, and 37-47% total coarse and very coarse, these size designations being defined in Table 2 herein. Preferred ranges of particle size distribution are: 40-44% total fines, 18-22% medium, and 38-42% coarse and very coarse.

All of the mesh sizes described herein are Tyler mesh sizes.

Having fully described the present invention it is to be understood that it is not to be limited to the specific details set forth, but is of the full scope of the appended claims.

I claim:

1. A fluid coke aggregate comprising ground calcined briquetted fiuid coke particles, said particles being bound together with a binder; said aggregate having a particle size distribution wherein 'B3-46% by Weight of the particles have a particle size less than about -65 mesh, 14- 26% O the particles have a particle size greater than 65 mesh and less than about 14 mesh and 37-47% by weight of the particles have a particle size greater than about -l-14 mesh, and less than 1A: inch, and wherein about 60-70% by weight of the particles having a particle size less than about 65 mesh have a particle size less than about 200 mesh.

2. The aggregate of claim l wherein said faggregate contains a paste binder in an amount to form an electrode therefrom upon baking.

3. The aggregate of claim 1 wherein said particles having a particle size less than about 65 mesh are present in an amount of about 40 to about 44% by weight, said particles having a particle size greater than 65 mesh and less than about 14 mesh are present in an amount of from about 18 to about 22%, and said particles having a particle size larger than about +14 mesh are present in an amount of from about 38 to about 42%.

4r. The aggregate ot claim 1 wherein said composition contains from about 1.5% to about 3% by weight of sulfur.

5. The aggregate of claim 2 wherein said binder comprises hard pitch.

6. An electrode prepared by baking the aggregate of claim 2.

7. An electrode comprising an aggregate of ground calcined briquetted fluid coke particles with paste binder, said aggregate being bound together in the electrode by a hard pitch, said aggregate having a particle size distribution wherein from about 33 to about 46% by weight has a particle size less than about 65 mesh, from about 14 to about 26% by weight has a particle size greater than 65 mesh and less than about 14 mesh and from about 37 to about 47% by weight has a particle size greater than about +14 mesh, and less than 1A inch, and wherein about Gti-70% by weight of the particles having a particle size less than about 65 mesh have a particle size less than about 200 mesh.

8. The electrode of claim 7 wherein said particles having a particle size less than about 65 mesh are present in an amount of from about 40 to about 44% by Weight, said particles having a particle size greater than mesh and less than about 14 mesh are present in an amount of from about 18 to about 22% by weight and said particles having a particle size greater than about +14 mesh are present in an amount of from about 38 to about 42% by weight.

References Cited UNITED STATES PATENTS 2,764,530 9/1956 Klemgard 204 294 X 3,009,863 11/1961 Angevine 204-294 X 3,197,395 7/1965 Nelson 204 294 JOHN H. MACK, PrimaryExaminer.

D. R. JORDAN, Assistant Examiner. 

1. A FLUID COKE AGGREGATE COMPRISING GROUND CALCINED BRIQUETTED FLUID COKE PARTICLES, SAID PARTICLES BEING BOUND TOGETHER WITH A BINDER; SAID AGGREGATE HAVING A PARTICLE SIZE DISTRIBUTION WHEREIN 33-46% BY WEIGHT OF THE PARTICLES HAVE A PARTICLE SIZE LESS THAN ABOUT -65 MESH, 1426% OF THE PARTICLES HAVE A PARTICLE SIZE GREATER THAN -65 MESH AND LESS THAN ABOUT -14 MESH AND 37-47% BY WEIGHT OF THE PARTICLES HAVE A PARTICLE SIZE GREATER THAN ABOUT +14 MESH, AND LESS THAN 1/4 INCH, AND WHEREIN ABOUT 60-70% BY WEIGHT OF THE PARTICLES HAVING A PARTICLE SIZE LESS THAN ABOUT -65 MESH HAVE A PARTICLE SIZE LESS THAN ABOUT -200 MESH. 